3v Motors on a 9v Battery

It's kind of taken for granted often that a 9v battery (thanks mixmar ;-) won't provide the current to drive a 3v hobby motor, especially when you run a microcontroller off of it too. Ignoblegnome and I were trying to drive that point home today in this post. However, CrazyCreator was insistant that the motor ran off of his battery. I got curious and rigged (essentially) the circuit you see in the image above. What it amounts to is holding one input low and one input as well as the enable on each motor channel high on a dual h-bridge to engage each channel at "full speed."

According to everything that every website and robot book in the world tells us, this shouldn't work. The cells in a 9v battery are just too small to provide enough current to drive a motor, let alone two of them, right?

Well, the experimental science says "Wrong!" Apparently, there's enough juice to spin up two standard yellow Dagu gearmotors for at least a minute or two.

This is not to say that it's a good idea, or that you'd be able to run more than a single servo of your UNO with a 9v battery as the sole power source (and I still don't think you could get even one motor to work on a 9v if it was used on a microcontroller too.) However, in this situation, the conventional wisdom is apparently wrong. Either that or Magnum Thunderbolt off-brand batteries pack a really cool surprise.

Since every video service I try to use can't seem to play nice with LMR today (at least on iPads,) here's a link to the video.

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My 4 leg insect bot is running 2 standard servos with a 9V rechagable battery. I learnedd that cheap no name servos does not move at all or just jitter when we tried to run them with a 9V battery. Maybe my higher quality servos are not drawing that much current.

Also my Chopstick Junior. He's running with just 4x AA Eneloop batteries all 8 servos and even the Arduino Nano...sometimes I even had an EL wire attached to it and running all of this from those 4x AA Eneloops...

So there is no general rule for this, just theories with numbers and equations. In real live everything can happen :-)

In theory it's right that the current is too low to fire up a 3V motor with a 9V battery but it should run anyway since the voltage is higher as the rated voltage of the motor.

Running a 6V servo on a 9V battery is not a good idea. Not because of the motor but because the control circuit can be damaged. The design of the control circuit will determine if it works or not at different voltages.

As for Chopsticks Junior, there can be a big difference in internal resistance between different brands of battery either by design or as a result of quality. Apparently the Eneloop are a good brand. I have never heard of them before. Are they a German brand?

Gotcha-that's not what I meant of course, but in an odd case of reality-mirroring-a-mistake, a 9v motor can provide the current to drive another motor if used as a generator, and as we've seen here, it would probably work on 3v motors!

I never doubted that a 9 volt battery could provide enough power to spin small motors, but it will be way outside its rated parameters, so it is a pretty bad idea.

Not only will the battery run out very quickly it will also be unable to provide it's rated voltge, which may brown out a microcontroller, and the motor's won't provide their rated torque (which in your case is pretty low anyway).